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If the edge is a bit rounded, the magic marker will only be rubbed off at the top of the bevel.

As for how long to get rid of the rounding, the only answer is as long as it takes.

If your knife...

IME sandpaper leaves the edge a bit beat up, which is why I never use it except for big nicks. One thing is it tends to round the edge a bit, so you have to hone past that on the 4k so it takes...